The U.S. government warned of a possible Internet attack on U.S. stock market and banking Web sites from a radical Muslim group, but officials said the threat was unconfirmed and seemed to pose no immediate danger. The notice was issued Thursday to the U.S. cyber security industry after officials saw a posting on a "Jihadist Web site" calling for an attack on U.S. Internet-based stock market and banking sites in December, said Homeland Security Department spokesman Russ Knocke.

There is no information corroborating the threat, Knocke said, adding that the alert was issued "as a routine matter and out of an abundance of caution. There is no immediate threat to our homeland at this time." Another government official said the threat had appeared on a Web site that called for Muslims to destroy American economic sites. The attacks were to be retaliation for the holding of Muslims at the U.S. prison at Guantanamo Bay, Cuba, which houses prisoners accused of ties to terrorist groups.

The attacks were to be conducted in December, "until the infidel new year," the site said, according to a U.S. government translation. It called for attackers to use viruses that can penetrate Internet sites and destroy data stored there. The alert was issued by the U.S. Computer Emergency Readiness Team. The team, a partnership between private industry and a number of government agencies, is based at the Homeland Security Department and warns Web sites about virus outbreaks and other Internet attacks.

The Securities Industry Automation Corp., a data technology subsidiary of the NYSE and American Stock Exchange, immediately went to work in trying to shore up Wall Street's vulnerability. The company formed SFTI, known in the industry as "safety," which acts almost as a traffic cop for data streams in case the system is overloaded. SFTI currently protects data transferred by all the major U.S. stock markets. Each stock market also has its own firewalls, many of which were beefed up after the attacks.